There are several reasons why some people's teeth grow in crooked,
overlapping, or twisted. Some people's mouths are too small for their teeth,
which crowds the teeth and causes them to shift. In other cases, a person's
upper and lower jaws aren't the same size or are malformed, resulting in either
an overbite, when there is excessive protrusion of the upper jaw, or an under
bite, when the lower jaw protrudes forward causing the lower jaw and teeth to
extend out beyond the upper teeth.

Most often crooked teeth, overbites, and under bites are inherited traits
just as the color of your eyes or size of your hands. Other causes of misaligned
bites are early loss of baby or adult teeth; improper fit of dental restorations
(for example, fillings or crowns); too small of a jaw to accommodate erupting
teeth;
gingivitis (gum disease); undue pressure on the teeth and gums; misalignment
of jaw after severe facial injury; tumors of the mouth or jaw; or common
oral health problems in children such as thumb sucking, tongue thrusting,
pacifier use beyond the age of three, or prolonged use of a bottle.